Water-borne soil resistant coatings

ABSTRACT

A soil resistant aqueous coating composition in which the binder consists of a blend of a low Tg polymeric dispersion and a high Tg polymeric dispersion. The PVC of the composition is less than the critical PVC and the Tg of the low Tg polymer dispersion is less than 0° C. and the Tg of the high Tg polymer dispersion is a least 35° C. and the volume ratio of low Tg polymer dispersion to high Tg polymer dispersion is from 0.4:1 to 1.4:1.

This application claims benefit of international applicationPCT/AU94/00600, filed Sep. 29, 1994.

TECHNICAL FIELD OF THE INVENTION

This invention relates to water-borne coatings and in particular tocoatings that exhibit good soil resistance properties.

BACKGROUND OF THE INVENTION

The appearance of coatings can be adversely affected by atmosphericpollutants such as dirt, dust and carbon particles from combustionprocesses being deposited on the surface of a paint film. For example, awhite paint film may become quite grey from atmospheric pollutants afterexterior use in major industrial cities. In some industrial urbanenvironments unacceptable soiling may take place within as short aperiod of time as several weeks. The problem of atmospheric soiling isobviously of greatest concern for exterior coatings.

In many cases the soiling of the paint film surface is not able to berectified by washing by natural rainfall or artificial means. It isbelieved this relatively permanent soiling is due to the soilingparticles becoming embedded in the paint film. This effect can beexacerbated by exposure in tropical climates where heat and highrainfall may contribute to softening of a thermoplastic paint film. Itis believed a rough surface contributes to dirt particles becomingentrapped. In practice, however, it appears that gloss and semi-glosspaints which contain high levels of thermoplastic polymer latex binderexhibit the most severe soiling problems, particularly under hot andhumid conditions.

Soil resistance needs to be distinguished from stain resistance which isa property more commonly associated with interior paints which areexposed to a range of domestic staining materials such as food, oil andbeverages. Indeed paints that are good for stain resistance may be poorfor soil resistance and vice versa. For example, water-borne semi-glosslatex paints generally are good for stain resistance and stain removalwhen used as interior paints. These paints have good mar resistance andwet and dry scrub performance and this enables the stained or dirtiedsurface to be cleaned by scrubbing without causing the treated area to"gloss up" and exhibit a gloss that is different from the surroundingarea. However, when used as exterior paints these same paints can havepoor soil resistance.

For exterior paints various methods have been proposed to improve theirsoil resistance. One method is to use a self cleaning paint film which,while it may become dirty to a similar extent to a standard paintsurface, cleans itself by the surface of the paint film being eroded onexposure to the combined effects of atmospheric oxygen, UV radiation andrainfall. This erosion exposes a new surface with clean appearance.These coatings are also known as self chalking paints. Whilst overcomingthe problems of soiling to a certain extent they suffer from the problemof still becoming soiled and remaining so until film degradation leadsto the apparent cleaning of the surface. As discussed previously,unacceptable soiling can take place in a matter of weeks, yet erosion bydegradation is a relatively slow process taking many months before itbecomes significant. Another obvious deficiency with such coatings isthat because the coating film is gradually being eroded the gloss of thefilm is reduced with time and also more frequent repainting is required.They are thus less suitable in gloss and semi-gloss formulations.

Another approach to achieving soil resistant exterior coatings has beento modify the surface of a coating film so that the adhesion of soilingparticles to the surface is reduced. In an article "A Soil ResistantTreatment For Low Gloss Coatings" by D. L. Gauntt et al, Journal ofCoating Technology, Vol 63 No. 803 page 25 December 1991, a treatmentsuitable for use on military aircraft is described. This article detailsthe use of an unpigmented colloidal silica treatment to be applied to anexisting paint film. It is proposed that good soil resistance isachieved in practice because the surface charge of the resultant film ischanged such that atmospheric particles are less strongly attracted andadhered to the surface, thus enabling much easier clean up. Whilstpromising results are reported in this article such systems suffer fromthe obvious disadvantage of requiring an additional coating to beapplied and furthermore this teaching is directed to low gloss coatingsbecause of the use of colloidal silica extenders.

A further approach to dirt pick up resistant coatings is disclosed in EP0 522 789 A2. This specification relates to high film build elastomericcoatings. Improved soil resistance is achieved by combining polymers ofdifferent Tg's within the one polymeric dispersion particle, providingan outer shell of harder polymer around a softer inner polymer core, andincluding photochemical crosslinking. These coatings are promoted forhigh build applications and are not recommended for use at the lowerfilm builds of conventional paints. When used at these lower film buildsdirt pick up is reported in Rohm and Haas promotional literature to bepoor.

Australian patent application AU-A-80149/91 by Rohm and Haas Companyrelates to film forming binders which are a blend of emulsion polymersand paints formulated from such binders. The blends consist of a leasttwo emulsion polymers, a hard emulsion polymer having a Tg greater than20° C. and a soft emulsion polymer having a Tg less than 20° C. Thecompositions described in this Rohm and Haas specification are claimedto be particularly useful in providing good block resistance whenformulated as interior semi-gloss paints whilst retaining good filmformation properties at low temperatures. No statement is made aboutsoil resistance or use on exterior surfaces. At page five of thisspecification the inventors explain that if a soft emulsion polymer isblended with increasing amounts of a hard emulsion polymer, the minimumfilm forming temperature (MFFT) of the blend will remain about equal tothe MFFT of the soft polymer until the hard polymer is about 50 weightpercent of the polymer blend. As it is important for their compositionsto have good low temperature film coalescence the preferred bindercompositions have 60% by weight soft and 40% by weight hard polymer. Intheir examples the soft to hard latex polymer ratio varies from 4:1 to1.5:1 and the soft polymers are said to have a MFFT greater than 0° C.In view of the high level of soft polymer these compositions when usedas exterior coatings would not be expected to have good soil resistanceproperties. The inclusion of the hard latex is believed to provide thegood block resistance properties desired for interior semi-gloss paints.The property of soil resistance does not necessarily correlate with filmhardness and paints of equal hardness can demonstrate significantlydifferent soil resistance.

U.S. Pat. No. 5,308,890 of Snyder published 3 May 1994 discloses coatingcompositions consisting of blends of a multistage core-shell dispersionpolymer and a non-film forming dispersion polymer. This patent isdirected towards obtaining harder films which are coalescent-free.Again, as for the previously discussed Australian patent application thepreferred and exemplified embodiments demonstrate that beneficialproperties are achieved when the weight percentage of the non-filmforming dispersion polymer is from 20 to 40% of the binder composition.Whilst the examples illustrate improved hardness and associatedproperties, they do not mention soil resistance which would be predictedto be poor in view of the high level of soft polymer.

Japanese Patent J 5 9215-365-A in the name of Nippon Acryl Kagakudiscloses compositions obtained by mixing 35-45 wt % acrylic resinemulsion of Tg 25 to 50 degrees C. and 55-65 wt % acrylic resin emulsionof Tg -10 to +15 degrees C. These compositions thus have a soft to hardratio of 1.9:1 to 1.2:1. These compositions also have a plasticiserwhich may be volatile. Whilst these paints have particular applicationas ventilator coatings to prevent the smearing of the surface, anti-soilproperties are not examined, though they would be expected to be poor.

U.S. Pat. No. 4,385,152 of Boyack et al discloses emulsion coatings forcoating uncured cementitious substrates. The compositions consist of ablend of a multi-stage first polymer dispersion and a second polymerdispersion. The multi-stage polymer dispersion has a hard core and softouter shell. It is a requirement for this invention that both polymerdispersions are film forming at ambient temperatures. Blends of thesetwo dispersions are claimed to lead to improved dirt pick up resistancewhen applied to cementitious substrates.

U.S. Pat. No. 4,069,186 of Ramig discloses paint compositions havingfilm forming latex binder, non-film forming plastic polymer particlesand opacifying pigment. These paint compositions have apigment-volume-content (PVC) greater than the critical PVC as measuredby opacity. Indeed it is an object of that invention to provide paintswith improved opacity whilst maintaining excellent film integrityproperties. The paints are intended for use as interior low gloss highopacity paints. Preferred paints are formulated such that the PVC isgreater than the critical PVC, as measured by opacity, but less than thecritical surface porosity PVC, as measured by enamel hold out. Thenon-film-forming particles used in this prior art invention areexplained at column 6 line 61, as being preferably polystyrene andindeed this is consistent with the examples which are limited topolystyrene, polymethylmethacrylate and polyvinylchloride. Most of theexamples use polystyrene particles which is consistent with the intendedinterior application for these coatings where the relatively poorexterior durability of polystyrene would not be relevant. Notsurprisingly, in view of the intended application, no testing asanti-soil exterior paints is mentioned.

U.S. Pat. No. 4,283,320 of Carroll et al has the same assignee as thepatent of Ramig and discloses opacified latex semi-gloss paintcomprising film forming latex binders combined with minor amounts ofopacifying pigment and substantial amounts of non-film-forming polymericparticles. The compositions as defined in claim 1 include 55 to 70% offilm-forming polymer particles. Accordingly if the non-film-formingparticles are regarded as hard particles the ratio of soft to hardparticles is from 7:1 to 2.2:1 in this prior art invention. The Tg ofthe film forming particles is at least 5° C. for ordinary roomtemperatures. Again no testing of the compositions as soil resistantexterior coatings is reported but they would be expected to be poorbecause of the large proportions of soft polymeric particles.

SUMMARY OF THE INVENTION

This invention provides in one form an aqueous coating compositioncomprising a blend of a low Tg and a high Tg aqueous polymericdispersions characterised in that: the PVC of the coating composition asmeasured by opacity is less than the critical PVC, the polymerdispersion with low Tg has a Tg less than 0° C., the polymer dispersionwith high Tg is non film forming and has a Tg of at least 35° C. and thevolume ratio of low Tg polymer dispersion to high Tg polymer dispersionis from 0.4:1 to 1.4:1.

Preferably the volume ratio of low Tg polymer dispersion to high Tgpolymer dispersion is from 0.6:1 to 1.2:1 and more preferably 0.7:1 to1.1:1.

DETAILED DESCRIPTION OF THE INVENTION

A commonly used characterising parameter for latex paints is thepigment-volume-content (PVC) which is the percent by volume of pigmentpresent in the dried paint film relative to the total volume of thedried paint film. The term pigment includes mineral opacifiers such asTiO₂ and ZnO as well as extender particles and non deformable "plastic"pigments. At a certain PVC, the volume of pigments in the paint filmabruptly causes considerable air voids in the dry paint film due tobinder deficiency. The PVC measurement at this level of pigment contentis characterised as critical PVC wherein a binder deficiency occurs suchthat insufficient binder is present in the paint film to encapsulate thepigment particles and fill any remaining voids in the film. At the onsetof such porosity in the paint film due to binder deficiency, opacity orhiding characteristics of the paint film increases abruptly. This pointis termed the critical-PVC as measured by opacity and is referred tohereinafter as the CPVC (opacity).

In coatings, as described in the present invention, the high Tg aqueousdispersion under some circumstances may contribute to the PVC of thepaint depending on the extent of deformability of the particles. Thecoatings of the present invention must have PVC's less than the CPVC(opacity) to achieve the required mechanical film properties. The CPVC(opacity) is determined experimentally by a PVC ladder. A PVC ladderseries of experiments leads to a graph relating the opacity of driedpaint films as a function of pigment-volume-content (PVC). The PVC isvaried by changing the relative volume of the binder by adding orsubtracting a given pigment component while maintaining the volumepercent of other pigment components constant. To determine whether thehigh Tg dispersion of the present invention is acting as a "plasticpigment" the ladder series is developed by varying the relativeproportions of high and low Tg dispersions.

The actual PVC of the compositions of the present invention may bedifficult to calculate as the extent of deformability of the high Tgdispersion may change. However the practical CPVC (opacity) may bemeasured by increasing the proportion of high Tg dispersion relative tothe low Tg dispersion to ascertain the CPVC (opacity). The coatingcompositions of the present invention are always below the CPVC(opacity) and this can be established experimentally.

The aqueous dispersions, both high and low Tg of this invention areusually copolymers of addition polymerisable ethylenically unsaturatedcomonomers. Examples of useful comonomers include alkyl acrylates andmethacrylates where the alkyl group has 1 to 20 (and more preferably 1to 8) carbon atoms, styrene, vinyl acetate, acrylonitrile and simple orsubstituted olefines such as butadiene and ethylene. However, as themajor application for soil resistant compositions is as exterior paints,monomers of known good exterior durability are preferably used. Forexample, monomers such as methyl methacrylate and butyl acrylate andsimilar alkyl acrylate and methacrylate esters are preferred. Monomerscontaining other functional groups such as hydroxyl or carboxyl groupsmay be used. Minor amounts of functional monomers such as ureido, amineor acetoacetate containing monomers directed towards specific objectivessuch as adhesion promotion or crosslinking may also be incorporated. Theselection of comonomers and their relative proportions is based on wellestablished principles in the art and generally balance properties suchas exterior durability, toughness and hardness with cost. Preferredcompositions include acetoacetate functional groups, particularlyprepared by copolymerising acetoacetoxyethyl methacrylate (AAEM)monomer, preferably at least 0.5% and more preferably at least 2.0%.

The molecular weight of the copolymers is generally similar to knownlatexes and typically would be at least 25,000 (Mn). A gelling structurein the particles is permitted in a similar fashion to the gel structureof known commercial latexes. In some cases it is desirable to enhancethe gel structure by including minor amounts of polyfunctional acrylateor methacrylate monomers provided film formation is not unacceptablycompromised.

The aqueous dispersions of the present invention may be prepared byknown means, for example emulsion polymerisation. Good results areachieved when the aqueous dispersion is prepared such that it issterically stabilised. The term sterically stabilised is an artrecognised term in colloid science and means that, in aqueous systems,colloidal stability is conferred by the presence of a sheath of watersoluble polymeric or uncharged oligomeric chains anchored to theparticle surface. By sterically stabilised dispersions we mean thatwhilst other types of stability, such as anionic or cationic, may bepresent there is also colloidal stability conferred by stericstabilisation. For the purposes of this invention a convenient test toascertain whether the dispersion is sterically stabilised or not is toincrease the ionic concentration of the aqueous phase to such an extentthat ionic stabilisation is nullified and then to assess the stabilityof the dispersion.

Examples of the preparation of dispersions suitable for the working ofthe present invention are described in Australia Patent No. 618,176.Also within the scope of the present invention are polymer dispersionswhich have a core/shell structure whereby either the hard particles orsoft particles or both are structured so that the outer shell of theparticle differs in polymer composition from that of the centre of theparticle. Polymer particles that include voids are also within the scopeof the present invention.

The aqueous dispersions usually give thermoplastic films. However withinthe scope of the present invention are crosslinkable polymer dispersionswhere cross-linking takes place after film formation. Examples are wellknown in the art and particularly useful examples are described in ourcopending Australian patent application entitled "Crosslinkable AqueousCoating-Compositions" No. PM1525.

An important characteristic of the two aqueous dispersions thatcontributes to the working of the present invention is their glasstransition temperatures (Tg). The term "glass transition temperature" isa term well known in the art and generally defines the onset of longrange molecular motion wherein the polymer preserves the outwardappearance of a solid but becomes rubbery and then tacky with increasingtemperature and undergoes plastic flow and elastic deformation. Thetheoretical Tg is often calculated in accordance with the Fox equation1/Tg=Wi/Ti. However, the glass transition temperatures are bestdetermined by measurement. Glass transition temperature can be measuredin accordance with the differential thermal analysis method set out inthe Journal of Paint Technology, Volume 41, pages 167-168 (1969) or bytesting for softening points, using a minimum film forming temperaturegradient bar or thermomechanical analysis (TMA) or differential scanningcalorimetry (DSC). In practice we find in most cases that the Tg asdetermined by different methods is close to the theoretical Fox Tg.However in some cases significant variations can occur and in thoseinstances we prefer to use Tg as measured on each dispersion, eitherduring film formation using a thermal gradient bar (MFFT), by DSC or byTMA measurements on dried films. Our preferred method is by TMA.

The low Tg dispersion should have a Tg of 0° C. maximum, although betterfilm properties are achieved when the Tg is -60° to -5° C. and morepreferably -20° to -10° C.

It is surprising that soil resistant coatings can be based on such a lowTg polymer as it would seem that a Tg lower than normally used in acoating composition, 5°-15° C., would lead to coatings that are poor forsoil resistance as dirt particles would be expected to be more readilyembedded in the surface. Not only are soil resistant coatings obtainedin the present invention despite using unusually low Tg latex particlesbut in most applications coatings can be achieved that do not requirethe addition of volatile plasticisers. As paints containing normallevels of volatile coalescents tend to be poor for soil resistancepreferred paint compositions are free of organic solvents andcoalescents. Accordingly coatings can be prepared that comply with thezero VOC requirement for low pollution paints.

The high Tg non film forming dispersion of the present invention has aTg of at least 35° C., preferably 40°-110° C., and more preferably45°-90° C. and most preferably 50°-70° C. There should be a differenceof at least 35° C., preferably 45° C., and more preferably at least 55°C. between the Tg's of the two polymeric dispersions. It will beappreciated that the upper limit in Tg is also influenced by therequirement that the PVC of the coating is below the critical PVC asmeasured by opacity. For example, if the high Tg dispersion comprisedpolystyrene particles, paints formulated at or above the CPVC (opacity)as in the US patent of Ramig previously discussed are not included inthe coatings of the present invention. In this specification the termnon film-forming means that at 20° C. a wet film cast on a substratefails to form a continuous film.

The relative proportion of the low and high Tg dispersions is mostimportant to the working of the present invention and we require thatthe volume ratio of low Tg polymeric dispersion to high Tg polymericdispersion to be from 0.4:1 to 1.4:1. This is of course calculated on anon-volatile basis. In practice there is usually little differencebetween the weight and volume ratios. If the particles are based on thesame monomers it is convenient to use the weight ratios as the densitiesof the particles are essentially the same.

At ratios above the 1.4:1 upper limit the soil resistance of paint filmsbecomes unacceptably poor. As is demonstrated in later comparativeexamples soil resistance at a ratio of 1.5:1 is very poor. At ratiosbelow 0.4:1 film properties such as ability to coalesce satisfactorilyat low temperatures is adversely affected.

The compositions of the present invention are particularly suitable forgloss and semi-gloss paints. In conventional latex paints the low PVC ofthese type of coatings can lead to poor soil resistance. For these typesof paints we have found it to be advantageous to use small particle sizepolymer dispersions, especially for the high Tg polymer dispersion. Theparticle size of both the low and high Tg particles should preferably beless than 200 nm in diameter, more preferably less than 150 nm and mostpreferably less than 100 nm. Preferred paints have a gloss of at least10 (60° head) and more preferably 40 (60° head).

Whilst the major portion of the binder in this invention is thecombination of a low Tg and high Tg polymer dispersion as describedabove, other binders may be included. If a third polymeric dispersion isincluded it may be regarded as a high or low Tg dispersion if its Tgmeets the criteria set out above. For example within the scope of thepresent invention are binders of polymer dispersions of -15° C. (40%),-10° C. (10%) and +50° C. (50%). In this case both the -15° C. and -10°C. dispersions are regarded together as the low Tg, and in this case thelow Tg to high Tg ratio is 1:1. Furthermore minor amounts of dispersionsthat are neither low or high Tg as defined in this invention may be usedas well as minor amounts of solution polymers and other additives asused in art recognised quantities.

Unpigmented or clear coatings and pigmented stains are also embracedwithin the scope of the present invention. Normally the PVC of pigmentedcoatings is approximately 20 to provide the required level of gloss andopacity. In the case of unpigmented coatings we have found the samebeneficial soil resistance properties. In practice, for best results,slightly higher levels of high Tg particles are used for clears.Although soil resistant properties are of most advantage in exteriorcoatings, the compositions of the present invention also haveapplication as interior paints.

The invention will be further described by reference to the followingexamples of preferred embodiments.

EXAMPLE 1

This example illustrates the preparation of a semi-gloss paint accordingto the invention and its evaluation.

A. Preparation of aqueous dispersion of Fox Tg -15° C.

    ______________________________________                                        Material               Wt %                                                   ______________________________________                                        A      Fatty alcohol ethoxylate surfactant*                                                              1.265                                                     Water               7.406                                                     t-butyl perbenzoate 0.380                                                     30% hydrochloric acid                                                                             trace                                              B      Ferrous sulphate    0.009                                                     Water               0.940                                              ______________________________________                                         *As described in Example 3 of International Patent Application No.            PCT/AU90/00565.                                                          

    ______________________________________                                                       Feed 1  Feed 2  Feed 3                                         ______________________________________                                        C     Methyl methacrylate                                                                          2.08      3.66  11.53                                          Butyl acrylate 3.32      6.16  19.37                                          Adhesion promoting                                                                           0.18      0.28  0.89                                           monomer*                                                                      Fatty alcohol ethoxylate                                                                     0.76      0.49  1.02                                           surfactant                                                                    Water          5.15      7.86  24.94                                    D     Sodium Erythorbate                                                                           0.03      0.04  0.14                                           Water          0.28      0.44  1.39                                     ______________________________________                                         *1-{2- N-(2-hydroxy-3-methacryloxypropyl)-μ(2                              hydroxybutyl)!aminoethyl}-2imidazolidinone (35% w/w in propylene              glycol/isopropanol) as described in Australian Patent 557,116.           

A stage was added to a vessel equipped with stirrer, condenser and feedfacilities and heated to 70° C. with stirring under a nitrogen blanket.

D stage was made up as a stock solution. Feeds stages C1, C2 and C3 wereeach premixed and separately emulsified under high shear and D stageadded. B stage was added to A stage and stirred for 10 minutes prior tostarting C stage feeds. C1 was fed over 50 minutes, C2 over 45 minutesand C3 over 120 minutes maintaining the reaction temperature at 70° C.At the end of the feeds, the reaction mixture was allowed to cool toroom temperature. The solids of this dispersion was 51% by weight andthe particle size (Dn) was 80 nm. It had a MFFT <0° C. (crack point) andTMA Tg of -16° C. This dispersion was designated dispersion 1A.

B. Preparation of aqueous dispersion of Fox Tg +70° C. The method ofpreparing dispersion 1A was repeated except that the C stage feeds wereas follows.

    ______________________________________                                                       Feed 1  Feed 2  Feed 3                                         ______________________________________                                        C.    Methyl methacrylate                                                                          5.27      8.47  26.38                                          Butyl acrylate 0.84      1.35  4.05                                           Adhesion promoting                                                                           0.18      0.29  0.90                                           monomer                                                                       Fatty alcohol ethoxylate                                                                     0.77      0.50  1.01                                           surfactant                                                                    Water          5.18      7.76  25.01                                          Sodiuin erythorbate                                                                          0.03      0.04  0.14                                           Water          0.28      0.44  1.39                                     ______________________________________                                    

This dispersion had a solids content of 51% and a particle size of 91nm. It had a MFFT of 60° C. and TMA Tg of 54° C. It was designateddispersion 1B.

C. Preparation of semi-gloss paint having a ratio of low Tg to high Tgpolymeric dispersion of 0.8 to 1.0.

    ______________________________________                                        Material                  Wt %                                                ______________________________________                                        A     Water                   8.000                                                 Proxel GXL (biocide)    0.080                                                 Bevaloid 4226 (antifoam)                                                                              0.200                                                 Triton X405 (octyl phenol ethoxylate                                                                  0.122                                                 surfactant)                                                                   AMP95 (amino methyl propanol dispersant)                                                              0.244                                                 Zincweiss 2011 (zinc oxide pigment)                                                                   3.997                                                 RTC90 (titanium dioxide pigment)                                                                      23.170                                          B     Water                   9.888                                                 Dispersion blend (45% 1A and 55% 1B)                                                                  52.357                                          C     Acticide EP (fungicide) 1.000                                                 75% QR 708 in ethanol (associative thickener)                                                         0.940                                                 (ex Rohm and Haas)                                                      ______________________________________                                    

The materials from stage A were added sequentially with stirring andthen under high speed stirring were dispersed for 10-15 minutes toachieve good pigment dispersion.

The millbase was gradually let down with B stage water, the dispersionblend from stage B was then added and the mixture stirred for 10minutes. Fungicide from Stage C was added and then the thickener wasgradually added in a pencil thin stream, the stirrer speed beingincreased as the viscosity increased. The paint was stirred at amoderate speed for a further 10 minutes to give a paint with volumesolids 40%, weight solids 54.4, PVC 21%, WPL of 1.325 and pH 8.5. Thispaint was designated paint IC. This paint was tested and found to bewell below the CPVC (opacity).

D. Evaluation of the paint 1C.

Paint 1C (two coats) was applied to cement sheet and exposed on afactory roof at 45° to the horizontal for three months in Kuala Lumpur,Malaysia. The initial gloss was 27 (60° head). After this period thepanel was visually assessed for dirt pick up and rated as good relativeto a conventional premium low sheen exterior latex paint which was ratedas fair.

Example 2

This example illustrates the application of the invention to asemi-gloss paint where the ratio of low Tg to high Tg polymericdispersion is 1 to 1.

A paint was prepared as for paint 1C except that the ratio ofdispersions 1A and 1B was 1 to 1. This paint was tested and found to bebelow the CPVC (opacity). This paint when tested as in 1D had an initialgloss of 30 (60° head) and was rated as good for soil resistance.

Examples 3-9

These examples illustrate the use of a polymer dispersion that includesacetoacetate monomer as the adhesion promoting monomer and which includea range of different Tg's for the high Tg dispersion and which alsoinclude a number of ratios of low Tg to high Tg dispersions.

A Preparation of an aqueous dispersion of Fox Tg -14° C. The materialsand method of 1A were repeated except that C stage was as follows:

    ______________________________________                                                       1       2      3                                               ______________________________________                                        C.    Methyl methacrylate                                                                          2.13      3.43 10.81                                           Butyl acrylate 3.74      6.02 18.94                                           Adhesion promoting                                                                           0.31      0.50 1.57                                            monomer*                                                                      Fatty alcohol ethoxylate                                                                     0.77      0.50 1.01                                            surfactant                                                                    Water          5.23      7.93 24.27                                     ______________________________________                                         *Acetoacetoxyethyl methacrylate                                          

This dispersion had a particle size of 90 nm and was designated 3A. TheMFFT was <0° C. and the TMA Tg was -12° C.

B Preparation of an aqueous dispersion of Fox Tg +67° C. The materialsand method of 1B were repeated except that C stage was as follows:

    ______________________________________                                                       1       2      3                                               ______________________________________                                        C.    Methyl methacrylate                                                                          5.44      8.31 26.15                                           Butyl acrylate 0.75      1.14 3.60                                            Adhesion promoting                                                                           0.31      0.50 1.57                                            monomer*                                                                      Fatty alcohol ethoxylate                                                                     0.77      0.50 1.01                                            surfactant                                                                    Water          5.31      7.97 24.66                                     ______________________________________                                         *Acetoacetoxyethyl methacrylate                                          

This dispersion was designated 3B and had a particle size of 95 nm. Ithad a MFFT of >60° C. and a TMA Tg of 52° C.

C. Preparation of aqueous dispersions of Tg's +30° C., +50° C., +90° C.

The process of preparing dispersions 3A and 3B was repeated except thatthe relative proportions of methyl methacrylate and butyl acrylate wereadjusted in turn to provide theoretical (Fox equation) Tg's of +30° C.,+50° C. and +90° C. These dispersions each contain 5% AAEM as fordispersions 3A and 3B. The dispersions of Tg's +30° C., +50° C., +90°C., were designated 3C, 3D and 3E respectively. The respective particlesizes were 93 nm, 83 nm and 106 nm. The TMA Tg's were 49° and 82° C. fordispersions 3D and 3E.

D. Paints were prepared as in Example 1 using the ratios of low Tg tohigh Tg dispersions as shown in the table below. In all examples the lowTg dispersion was 3A with a Fox Tg of -14° C. Results for soilresistance when assessed as in Example 1 are also shown. All paints weretested and found to be below the CPVC (opacity). Those within the scopeof the present invention exhibit an excellent balance of properties.Satisfactory soil resistance was also exhibited by a paint prepared fromdispersions 3A and 3B at a ratio of low Tg to high Tg of 1.2:1.

    ______________________________________                                                            Ratio of                                                                      low Tg to       Soil                                      Example Tg of high Tg                                                                             high Tg   Gloss Resistance                                No.     dispersion  dispersion                                                                              (60°)                                                                        Rating                                    ______________________________________                                        3       3E(+90° C.)                                                                        0.8:1      9    Very Good                                 4       3B(+67° C.)                                                                        0.8:1     30    Very Good                                 5       3D(+50° C.)                                                                        0.8:1     36    Very Good                                 6       3D(+50° C.)                                                                          1:1     38    Good                                       7*     3C(+30° C.)                                                                        0.8:1     51    Poor                                       8*     3E(+90° C.)                                                                        2.3:1     19    Fair                                      9       3D(+50° C.)                                                                        0.7:1     35    Very Good                                 ______________________________________                                         *Comparative examples, not according to the invention.                   

Example 10

This example illustrates the good soil resistance properties accordingto this invention when formulated as a gloss paint.

A. A gloss paint was formulated and prepared as below.

    ______________________________________                                        Material                  Wt %                                                ______________________________________                                        A     Water                   6.000                                                 Proxel GXL (biocide)    0.080                                                 Bevaloid 4226 (antifoam)                                                                              0.200                                                 Triton X405 (octylphenol ethoxylate surfactant)                                                       0.107                                                 AMP95 (amino methyl propanol dispersant)                                                              0.214                                                 RTC90 (titanium dioxide pigment)                                                                      23.170                                          B     Water                   12.115                                                Dispersion blend        56.164                                                (50% dispersion 3A and                                                        50% dispersion 3D)                                                      C     Acticide EP (fungicide) 1.000                                                 75% QR 708 in ethanol (associative thickener)                                                         0.950                                           ______________________________________                                    

The materials from stage A were added sequentially with stirring andthen under high speed stirring were dispersed for 10-15 minutes toachieve good pigment dispersion.

The millbase was gradually let down with B stage water, the dispersionblend from stage B was then added and the mixture stirred for 10minutes. Fungicide from Stage C was added and then the thickener wasgradually added in a pencil thin stream, the stirrer speed beingincreased as the viscosity increased. The paint was stirred at amoderate speed for a further 10 minutes to give a paint with 40% volumesolids, PVC of 19%, pH 8.5, 53% solids by weight and a WPL of 1.282.This paint was designated 10A.

B. Paint 10A was evaluated as in Example 1, had an initial gloss of 57(60°) and was rated as good for soil resistance compared to a commercialpaint of similar PVC and gloss level which was rated as very poor.

Example 11

This example illustrates the preparations and testing of a gloss paintbased on polymeric dispersions prepared using anionic surfactants.

A. A polymeric dispersion of Fox Tg -13° C. was prepared as follows.

    ______________________________________                                        Material              Wt %                                                    ______________________________________                                        A      Water              43.558                                                     RK500 (Polyoxyethylene hexyl                                                                     0.900                                                      ether phosphate)                                                              Disodium orthophosphate                                                                          0.102                                                      Sodium hydroxide   trace                                               B      Methyl methacrylate                                                                              0.785                                                      Butyl acrylate     1.271                                                      Acetoacetoxyethyl methacrylate                                                                   0.108                                               C      Ammonium persulphate                                                                             0.136                                                      Water              0.370                                               D      Sodium 2-acrylamido 2-methyl                                                                     0.904                                                      propane sulphonic acid                                                        Water              5.941                                               E      Methyl methacrylate                                                                              15.626                                                     Butyl acrylate     25.257                                                     Acetoacetoxyethyl methacrylate                                                                   2.152                                               F      Fatty alcohol ethoxylate surfactant                                                              2.500                                               G      Water              0.113                                                      Sodium formaldehyde sulfoxylate                                                                  0.014                                               H      Water              0.113                                                      t-butyl perbenzoate                                                                              0.023                                               I      Water              0.113                                                      Sodium formaldehyde sulfoxylate                                                                  0.014                                               ______________________________________                                    

A stage was loaded to a reaction vessel equipped with stirrer, nitrogenblanket and feed facilities. The charge was heated to 75° C. B stage wasthen added with stirring and the pH was adjusted to 9. C stage was addedand at the completion of the exotherm D and E stages were fedconcurrently over four hours, the temperature being maintained at 75° C.throughout. F stage was added during the final 15% of E stage. Thereaction charge was maintained at 75° C. for a further fifteen minutesat the conclusion of D, E and F stages and then stages G, H and I wereadded at 10 minute intervals.

The solids of this dispersion (10A) was 49.5%. The particle size was 100nm, MFFT <0° C. and TMA Tg was -8° C.

B. A polymeric dispersion of Fox Tg +53° C. was prepared as for 11Aexcept that stages B and E were as follows

    ______________________________________                                        B       Methyl methacrylate                                                                             1.633                                                       Butyl acrylate    0.424                                                       Acetoacetoxyethyl-methacrylate                                                                  0.108                                               E       Methyl methacrylate                                                                             15.626                                                      Butyl acrylate    25.257                                                      Acetoacetoxyethyl methacrylate                                                                  2.152                                               ______________________________________                                    

After polymerisation this dispersion 11B had a solids content of 49.5%and particle size of 88 nm. The MFFT was >60° C. and the TMA Tg was 53°C.

C. A gloss paint was prepared as in Example 10 with 50% each ofdispersions 11A and 11B. This paint when tested as in Example 10 wasrated as very good for soil resistance. The paint had an initial glossof 62 (60° head).

Example 12

This example illustrates the use of a crosslinkable binder:

A semi-gloss paint was prepared as in Example 1 using dispersions 3A (Tg-15° C.) and 3B (Tg +67° C.) at a ratio of 0.8 to 1, except that 0.9parts of the combined dispersions (non-volatiles) was replaced with 0.9parts of Jeffamine ED600 crosslinker, which was added as part of Bstage. This level of amine crosslinker provided a mole ratio ofacetoacetate to NH₂ of 2 to 1.

This paint when tested as in Example 2 had an initial gloss of 31 (60°)and was rated as very good for soil resistance.

Example 13

This is a comparative example which is outside the scope of the presentinvention.

A semi-gloss paint was prepared in accordance with Example 1 except thatthe low Tg dispersion (1A) was replaced with equal parts by weight of adispersion of Tg +12° C. This paint required the addition of 2% Texanolcoalescent to overcome film formation problems. This paint was rated asbeing very poor for soil resistance.

Example 14

This is a comparative example which is outside the scope of the presentinvention.

A semi-gloss paint was prepared in accordance with Example 1 using a Fox0° C. Tg dispersion as the low Tg polymer dispersion and a Fox +70° C.Tg dispersion as the high Tg polymer.

This paint cracked severely and was incapable of forming a useful filmat low temperatures (7° C.) and was therefore not tested for soilresistance.

Examples 15-18

Examples 15 and 16 illustrate improved soil resistance in unpigmentedclear coatings according to the present invention. Examples 17 and 18are comparative examples outside the scope of the present invention.

Clear coating compositions were prepared by combining low Tg (-15° C.)acrylic polymer dispersion with a higher Tg (+60° C.) acrylic polymerdispersion in the ratios as set out below. Texanol** coalescent wasadded to obtain well coalescent films. The blends were applied over awhite substrate, exposed for 4 weeks in Kuala Lumpur, and measured forsoil resistance by reflectance.

    ______________________________________                                        Example  Texanol    Composition,                                                                              Soiling                                       No.      Coalescent low Tg:high Tg                                                                            Density++                                     ______________________________________                                        15       2.5%       0.54:1      1.63 (pass)                                   16       1.0%       0.8:1       2.37 (pass)                                   17*      --         1.5:1       3.95 (fail).sup.                              18*      2.0%       Revacryl 4176+                                                                            3.89 (fail).sup.                              ______________________________________                                         **Eastman Chemical Co                                                         +Commercial acrylic latex (Revertex)                                          *Comparative examples                                                         ++Calculation of Soiling Density                                              Soiling Density = 100 × Log.sub.10 (L*.sub.A /L*.sub.B)                 L*.sub.A = Initial L* value                                                   L*.sub.B = L* value at four weeks                                        

The smaller the value obtained for soiling density the better theresult.

L* value is the perception of lightness and darkness and is calculatedfrom measured reflectance as defined by the International Committee onIllumination in CIE publication No. 15 supplement 2.

These examples illustrate good soil resistance for unpigmentedcompositions within the scope of the present invention.

Example 19

This is a comparative example which illustrates the poor soil resistancewhen the low Tg to high Tg ratio exceeds 1.4:1.

Latexes where prepared in accordance with examples 3 and 4 AustralianPatent application AU-A-80149/91 by Rohm and Haas Company. Example 3 hada Fox Tg of -7.2° C., a TMA Tg of -1° C., a solids content of 47.7% anda particle size of 85 nm. Example 4 had a Fox Tg of 42.5° C., a TMA Tgof 55° C., a solids content of 46.7% and a particle size of 80 nm. Apaint was prepared and tested as in Example 10 above except 1.5% Texanolwas needed to obtain good film coalescence with a ratio of low Tg tohigh Tg particles of 1.5:1 and hence outside the scope of the presentinvention but a preferred ratio for the Rohm and Haas application. Thesoil resistance was rated as very poor. The 60° C. gloss of the paintwas 67.

Example 20

This example illustrates the use of a carboxylated acrylic terpolymerlatex as the low Tg polymer dispersion. A commercial latex (NationalStarch 278-6212) was used as the low Tg dispersion in Example 10. Thelatex had a solids content of 50%, particle size 200-300 nm and TMA Tgof -9° C. The supplier's literature value of the Tg was quoted as -15°C. The paint had a 60° C. gloss of 32 and after testing was rated asvery good for soil resistance.

Example 21

This example is within the scope of the present invention andillustrates the use of colloid stabilised latexes. Paints were preparedat a PVC of 21%, a volume solids of 41% with a ratio of low Tg to highTg dispersions of 0.7:1 using latexes prepared from a combination of PEGoleic acid monoester, sodium dodecyl sulphosuccinate and a cellulosederivative colloid. The paints included 2% Texanol plasticiser. Thecharacteristics of the latexes were as follows:

    ______________________________________                                                                        Tg    Particle                                         MMA    2-EHA   MAA     (DSC) Size                                    ______________________________________                                        low Tg dispersion                                                                        42.9     56.2    0.9   -5    554 nm                                (Fox Tg -18° C.)                                                       high Tg dispersion                                                                       74.8     24.3    0.9   62    566 nm                                (Fox Tg 40° C.)                                                        ______________________________________                                    

After evaluation as in Example 10 the soil resistance was rated as verygood. The 60° C. gloss of the paint was 20.

Examples 22-23

These examples are within the scope of the present invention anddemonstrate that good soil resistance may be obtained when latexesprepared as in Example 19 are used according to our invention. Paint wasprepared and tested as for Example 19 after including 5% of Texanol toobtain adequate film formation.

    ______________________________________                                                                60° gloss                                                                       Soil                                         Example No.                                                                             low Tg:high Tg                                                                              (initial)                                                                              Resistance                                   ______________________________________                                        22          1:1         71       fair/good                                    23        0.7:1         73       fair/good                                    ______________________________________                                    

Example 24

This example illustrates improved soil resistance in a storingcomposition used as a Coil Coating. A paint was made according toExample 10 using Examples 3A(Tg -15° C.) and 3E(Tg +90° C.) which wereblended at a ratio of 0.7:1. 3.5% of the solid latex was replaced withJeffamine ED600 (polyoxyalkylene diamine) to enable a crosslinked filmto be formed. The paint was drawn down on a primed Zincalume panel andpre-baked for 60 seconds @65° C. then baked to a peak metal temperatureof 232° C. in another 30 seconds. The paint had an initial 60° gloss of21, a pencil hardness of F and was rated as good for soil resistancecompared to a conventional polyester coil coating of similar PVC, glosslevel, the same pencil hardness and which was rated as very poor.

We claim:
 1. An aqueous coating composition comprising a blend of a lowTg and a high Tg aqueous polymeric dispersions wherein the PVC of thecoating composition as measured by opacity is less than the criticalPVC, the polymer dispersion with low Tg has a Tg less than 0° C., thepolymer dispersion with high Tg is non film-forming, has a Tg of atleast 35° C. and the volume ratio of low Tg polymer dispersion to highTg polymer dispersion is from 0.4:1 to 1.4:1.
 2. An aqueous coatingcomposition as defined in claim 1 wherein the volume ratio of low Tgpolymer dispersion to high Tg polymer dispersion is from 0.6:1 to 1.2:1.3. An aqueous coating composition as defined in claim 2 wherein thevolume ratio of low Tg polymer dispersion to high Tg polymer dispersionis from 0.7:1 to 1.1:1.
 4. An aqueous coating composition as defined inany one of claims 1-3 wherein the low Tg dispersion has a Tg in therange -60° to -5° C.
 5. An aqueous coating composition as defined inclaim 4 wherein the low Tg dispersion has a Tg in the range -20° to -10°C.
 6. An aqueous coating composition as defined in any one of claims 1-5wherein the high Tg dispersion has a Tg in the range 45°-90° C.
 7. Anaqueous coating composition as defined in claim 6 wherein the high Tgdispersion has a Tg in the range of 50°-70° C.
 8. An aqueous coatingcomposition as defined in any one of claims 1-7 wherein the particlesize of the aqueous dispersions is 200 nm maximum.
 9. An aqueous coatingcomposition as defined in claim 8 wherein the particle size of theaqueous dispersions is 100 nm maximum.
 10. An aqueous coatingcomposition as defined in any one of claims 1-9 wherein the aqueousdispersions include copolymerised acetoacetoxyethyl methacrylate (AAEM).11. An aqueous coating composition as defined in any one of claims 1-10further comprising opacifying pigment and having a gloss of at least 40(60° head).
 12. An aqueous coating composition as defined in claim 11that complies with zero VOC requirements for low pollution paints.
 13. Amethod of improving the soil resistance of a gloss or semi-gloss paintwith aqueous dispersion polymer binder by replacing the disperse polymerbinder by a binder as defined in any one of claims 1-10.